Simplified Stroboscopic System for Motion Pattern photography

The photography and study of subjects in motion has been one of the
fascinations of photographers since its invention. In fact, artists have dealt
with this same subject since the time humans started to make pictorial records
of their environment. With the development of photography, several workers devoted considerable
personal effort into perfecting motion recording techniques. These people
included Etienne Marey, Eadward Muybridge, Harold Edgerton, and others. Each
had their "favorite" way of approaching the problems associated with their
personal interests and all contributed greatly to our understanding of the
world of motion around us. Many of the techniques that were developed and applied for motion studies were
mechanical in nature as far as the photographic aspects of the techniques used.
Even motion picture cameras can be classified among these solutions for dealing
with recording and analyzing motion.

With the advent of the electronic flash and the electronic stroboscope, and
primarily under the guidance of Harold Edgerton from the 1930's through the
1980's, the recording of subjects in motion onto a stationary film (and less
well known, onto moving film) became almost the exclusive domain of electronic
stroboscopes.

Modern photographic stroboscopy in its simplest form is a method whereby a subject in
motion is lit by repeating flashes of light from the stroboscope while the
shutter of the camera remains open for a period of time long enough to capture
the subject in multiple locations during the time of exposure.

There are several factors that need to be considered, adjusted and
controlled to end up with technically effective photographs made in this
manner. These include a knowledge and the ability to adjust the flashing rate
of the stroboscope, the influence of the background on the final image, the
choice of exposure time and a knowledge of the output or power of the
individual flashes produced by the stroboscope.

I will describe the basic parameters at work in a typical stroboscopic set-up
by way of an example situation. Assume that the desire is to make a record of a golf club
swing or the swing of a tennis racquet (such as depicted in this example by
Harold Edgerton) or a gymnastic routine and that
this record must be a single image (as shown here) illustrating the motion as opposed to a motion picture.

The procedure is quite straightforward. One first locates a stroboscope.
This used to be fairly difficult as stroboscopes in the past were either too
weak for practical applications or quite complex and considered specialized
instruments and generally not available to non-specialists. These days the
situation is much improved and it is becoming easier to buy or rent
stroboscopes as flash manufacturers are even building a stroboscopic function
into their flashes. Also, the entertainment industry uses stroboscopes on a
regular basis to light up dance halls, etc.

For photography, the subject is placed in a suitable location and the camera
distance adjusted to provide a desired image in the viewfinder. It is advisable
to place the subject against as dark a background as possible since not doing
so will likely lead to the subject looking transparent in those areas that are
moving and thus which do not cover the same location in space throughout the
camera exposure. Since the background will be black it is advisable for the
subject to wear lighter colored clothing as dark parts of the subject may
otherwise blend into the background.

The light level is measured for a single flash of the stroboscope and based on
this information the lens aperture is adjusted appropriately based on the film
speed used. With a flash type stroboscope one generally is not concerned with
the exposure time of each flash, just the amount of energy or total light
discharged during a single flash. Some photographers prefer to underexpose
slightly for each flash so that wherever images overlap the density will not be
excessive on negative materials.

Now, after you have made sure that your subject can perform the action that you
want to portray in the allocated space, you open the camera shutter for a time
that is roughly dependent on the time at which your subject starts to move and
the length of time that elapses until the action you want to
track is completed. You can do this by
tripping the shutter set to a specific, estimated, exposure time or simply opening the
shutter on "B' at the beginning of the action and closing it when it is over.

Note that, as shown in the illustration recorded with a Colorado Video
Freeze Frame system, with every flash of the stroboscope some parts of the subject will
be in a different position and they will record on
different location on the film. The human eye/brain combination can easily
fill the gaps and make the connection from one image or position to the
next to generate an impression of what the action looked like in detail.

The separation between moving parts of the subject will be governed by the
frequency of the flash and the speed at which the subject moves. The faster the
flash rate the closer and more separate images you will get for a given
exposure time and the faster the subject moves the fewer images you will
capture between the beginning and end of the action.

Do-it-yourself stroboscopy

Light or flashing stroboscopes are interesting but usually out of reach of the
amateur's budget. I would like to suggest that a much more practical and
inexpensive way to get into stroboscopic photography is to investigate the
technique not with a flashing light but with a mechanical stroboscope.

A mechanical stroboscope consists of a rotating disc with a radial slot cut into it and the disc
placed in front of a camera's lens in such a manner that each time the slot
passes by the lens the camera sees the subject for a brief time interval.

This mechanical way to simulate a flash-type stroboscope is ideal for
experimenting and it is quite adequate for initial work with the
technique. In some cases it can yield more than adequate and
useful results. In fact, the mechanical stroboscope has certain advantages over
a flashing kind in that it can use the sun as a light source. This enables it
to deal with
large subject matter outdoors. The light flashing strobe can not be used
outdoors effectively since it would have to significantly overpower the light
from the
sun!

To make a mechanical stroboscope take a look at the illustration to the right. The
device can be even simpler than suggested here but I have found the following
construction to be practical and useful in many situations.

You will need a small DC (battery) powered motor that will run at a good clip
when attached to 6 to 9 volts DC. You will need to attach to it a slotted disc made
out of a thin sheet of black paper (construction paper will do nicely). You
will need a disc of stiffer material to act as a "protector" for the thinner,
slotted disc. This can be heavy, black, cardboard stock. You will also need a
step-up ring that will screw into your camera's filter thread to a filter size
that is possible 5 to 10mm larger then the filter size your camera lens takes.
So, if your camera takes 52mm filters you'd use an adapter ring that is maybe a
52 to 62 or even 52 to 72 mm in size. Finally, you will need a fairly stiff
piece material to which you will attach "protector" disc and the motor allowing the shaft of the
motor to protrude unimpeded through the cardboard disc.

You could make the larger, cardboard disc the device to which you also attach
the motor but I opted for a separate, wedge shaped, "foundation" for the stroboscope. This is
shown pictorially in the attached illustrations.

The step up ring is glued (use grey Epoxy glue that comes in 2 parts) to the
stiff, strong, base, wedge at a distance of about 4 inches or so from the
center of the motor shaft. A hole is cut in the support material allowing the
camera lens to see through the support wedge and the attached cardboard disc.

The cardboard disc has cut into it a second hole about 180 degree on the other
side of the hole where the camera lens "looks through". This second hole is
there to allow you to visualize what the camera is recording even while the
camera shutter is open and while the viewfinder of the camera is blocked.

The thin, construction paper, disc is then glued to the motor shaft at the
center. You may want to buy a flange of some kind that you can attach to the
motor shaft to make the gluing process easier than trying to glue to a narrow
metal shaft.

The size of the slot that you cut into the construction paper disc should be
about 10 degrees or so in size. The smaller it is the shorter the exposure times
it will deliver. The larger the longer and the smaller aperture or less light
you will need to use.

Once the stroboscope is assembled it is attached to the camera lens as shown in
the illustrations. The motor will have a couple of wires hanging from it and
when these are connected to a voltage source such a 9 volt transistor battery
the motor and the slotted disc will start to turn. You might make the connection
between the motor and the battery by way of a 9 volt battery clip.

I have found that I can power the motor using a "battery eliminator", a small
transformer that you can plug in the house current and that will deliver
adjustable voltages from 3, 4.5, 6, 7.5, 9 and 12 volts DC. The more voltage
you supply the motor the faster it turns.

As far as making a traditional stroboscopic record of an object in motion,
the procedures for using a mechanical stroboscope as opposed to a flashing one
are not much different. One big advantage of the mechanical device is that your
subject will be able to constantly see what it is that they are doing. With the
flashing light stroboscope subjects sometimes loose their balance and
coordination because it is easy to get confused while performing an action
under a discontinuous light source.

For photography the subject is again placed against a dark background and
when the photographer decides to start to record a sequence the shutter is
simply opened while the slotted disc turns in front of the lens, with each turn
of the disc making an exposure of the subject. The the end of the action the
shutter is closed and the film processed. The result will be a series of images
superimposed on each other where the subject did not significantly change
positions but showing the moving extremities of the subject in several different
locations in space. A true stroboscopic record.

To achieve proper exposure, one has to determine the approximate exposure
time delivered by the rotating slot. It can be determined by dividing the size
of the slot in degrees by the 360 degrees and multiplying this by the time it
takes the disc to make one revolution. Lacking an accurate measurement of the
time for one turn of the disc at a given voltage, it is probably appropriate to
estimate this time.

For example, assuming that the disc is turning maybe 5 times a second, the
time for one rotation is 1/5th second. Now if the slot measures 10 degrees,
then the light goes through to the camera for 10/360ths of the time it takes
the disc to make one turn. Or, 1/36th of 1/5th of a second. This is 1/180 of a
second. To determine the aperture the camera needs to be set to, use a light
meter to determine the aperture necessary for proper exposure given the film
being used and an exposure time of 1/180 second. Approximately.
time it

If you keep the shutter of the camera open for one second you will record 5
separate images of the moving subject. If the disc turns faster than 5 rps then
you will get more. If less then fewer. It is all determined by the relationship
between the number of times the slotted disc turns each second and the length
of time the shutter remains open!

Calibrating your rotating disc stroboscope

It is obvious that a general knowledge of the rotation rate of the
stroboscopic disc, and thus the frequency of photography, is useful to arrive
at predictable results. However, lacking suitable calibration and rotation
frequency measurement instruments leaves one with nothing but an estimation of the actual
stroboscopic frequncy of the device. This is better than nothing but there are
simple and inexpensive ways of calibrating the device.

One way to do this would be to photograph the rotating disc with a camcorder
particulary if one sets the shutter speed of the camcorder to a short exposure
time. The camcorder, upon frame by frame playback, will display images on the
TV monitor that are probably 1/60 second apart in time. It is a simple matter
to track the position of the slot on the disc to determine the number of
degrees that the slot turns between frames to arrive at the number of degrees
per second that the disc is turning. Dividing this by 360 will give you the
revolutions (or views) per second at the voltage that the stroboscope was being
driven at when the recodring was made.

This methods works but it assumes that one has ready access to a camcorder
and that the camcorder has a high speed shutter capability (most these days do).

Another, more "photographic", method is based on the fact, or realization of
the fact, that the shutters on most cameras are themselves highly accurate
timepieces. Also, it is useful to remember that a photograph is exposed for the
time that the camera's shutter is open. These two facts point the way for using
the very camera that the stroboscope will be mounted on as the instrument that
will measure the rotation rate of the stroboscope.

This calibration process, based on the "hints" given above,
can be done in several ways but the one that I am proposing here is to make an
exposure of some subject, such as a golf ball rolloing down an inclined ramp,
with the stroboscope operating at some given rate. The photograph should be
made at a reasonably long exposure time, maybe 1 second.

After the rolling ball has entered the field of view of the camera the
shutter (set to 1 second) is tripped. A second later the shutter closes. One
simply needs to make sure that the shutter closes before the rolling ball exits the
field of view of the camera on the other side.

After development it will be seen that the ball has left a "stroboscopic
track" of its path as it rolled down the inclined ramp. This path will be made up of a
number of separate or individual images of the ball since with each passage of
the rotating disc's slot a picture of the moving ball (in a diferent location
each time) was exposed on the film. This being the case, simply counting the
number of separate images give one a clear measure of the disc's rate of
rotation. If a one second exposure time was selected the rotation rate will be equal to the
number of images of the ball per second.

It will be useful to draw a "performance curve" plotting rotation rate
against voltage supplied to the motor so that in future photography sessions
one will have a fairly good idea of what voltage one will need to supply to the
motor to achieve a particular stroboscopic frequency. This will, of course,
depend on what subjects one will be faced with at later times.

Multiple Flash Based Stroboscope

While I would definitely argue that there is a lot to be said for keeping
things simple and reasonable in terms of cost, there is an alternative to a
mechanical stroboscope. It is based on the use of multiple flash units operated
in sequence.

The limiting factor that does not allow a standard electronic flash to fire in
rapid succession, as a stroboscope would, is generally the fact that the power
electronic flashes discharge per flash is quite high and it takes the circuit a
considerable time to recharge the main capacitors.

To fire a flash in rapid succession, therefore, it is necessary to use a flash
equipped with a thyristor switching circuit that allows the photographer to
operate the flash at partial power levels, such as 1/16 or less. Once a flash
is operated at 1/64th or 1/128th power generally the recycling time is reduced
to a second or less. This means that if one had a device that would trigger the
flash repeatedly one could conceivable operate the flash at a frequency of 1
flash per second and possibly even faster. This is not very high by stroboscope standards but it is a
place to start with.

Triggering a flash by hand at frequencies of one flash a second or faster is not too easy
and my suggestion is that one use an auxiliary device to simplify the switching
procedure. The device I am suggesting is what is called a rotary swith. These
are available at electronic parts supply stores. They are devices that close a
series of contacts sequentially as the shaft of the switch is rotated.

If you attach a female PC socket to one of the switches in the rotary switch
and a lever to the shaft, then it is a simple matter to rotate the shaft at a
steady speed and have the powered-down flash operate in repeatable fashion at a
steady rate. The shaft may be attached to something like a variable speed
electric drill for motorized operation.

Realizing that the frequency that the flash can fire is limited by the
recharging time, now it becomes obvious that one could increase the maximum
frequency achievable by a single flash by connecting multiple flashes to the
contacts of the rotary switch. For example, if one can achieve a maximum rate
of 1 flash per second with a single flash, by connecting 4 similar flashes to
4 switch contacts located at 90 degree intervals on the switch, and rotating
the shaft once a second, this will trip the 4 flashes sequentially at an
equivalent flashing frequency of 4 flashes per second. This is starting to
become interesting!

The flashes should usually be set to the lowest possible power they are
capable. One can
use automatic flash units by using a small reflector deflecting some of the light from the flash
head of a given unit to its light detecting window. This makes the flash quench
its flash very quickly and the result is that it operate at low power in
this fashion.

If you do build a "sequential stroboscope" such as described here additional
creative opportunities open up for the photographer. It is possible to vary
the frequency of the stroboscope by simply altering the rotation rate of the
shaft of the switch. It is also possible to place different colored filters
over each flash and have these provide a multicolored sequence.

Finally a note of warning and a disclaimer. Please note that even with this
procedure most electronic flashes are not designed to operate in continuously
repeating mode for an extended time so please take the above suggestion with a
good dose of caution by not operating the flash or flashes for more than a few
seconds in repeating mode. The author can not be held liable for damage
resulting to your flash unit if you attempt this method for simulating a
stroboscope. The reason for this is that experimental conditions can vary
widely and not all situations can be predicted from the author's position.

Stroboscopic photography with digital cameras

Generally one is interested in photographing relatively long duration
events (even though one may only be talking about a second or two!) because it
is really pointless to to make a stroboscopic record while only recording the
subejct in two or three positions over time!

For example, to make a record of a golf swing or something similar, we would
be looking to make a record over a time period of a second or so. During that
time we might want to record our subject in maybe 20 to 100 different
positions. This, of course would require a strobe flashing at a frequency of 20
to 100 flashes per second if we kept the shutter open for a second. An exposure
time of 1 second is easy to accomplish with a regular camera but many digital cameras have a
limited maximum exposure time. Sometimes this is as short as 1/8 or 1/4 second.
This means that making stroboscopic records with such cameras is, in a way,
limited to events of relatively short duration.

Well, recently I attached a mechanical stroboscope to a Kodak DC260 camera
(which has a maximum exposure time of 4 seconds) and used an exposure time of one second to obtain the photograph shown here.
The subject was placed against a large, black, velvet background. The lighting
level was adjusted so that the results obtained were of an acceptable quality
by making a few preliminary tests and judging the quality of the images on the
LCD display screen of the camera. Very convenient and effective!

By the way, from this example made over a period of 1 second, the camera
recorded about 17 separate images of my amateurish attempt to swing a glof club. From
this it can be determined that the stroboscopic disc was turning at about
17 revolutions per second.

Note also that, as with traditional cameras, the background (which should have
reproduced very dark since it was black velvet) and those parts of my
body that remained essentially in the same position appear significantly
overexposed because they reflected light to the same location on the CCD, while
the moving club shaft was exposed in different positions on the CCD with every
pass of the stroboscope disc. The moving club, therefore, is exposed only once
on any given area on the CCD. One could improve slightly on the tonal range of
the image by making the subject wear dark clothing while painting the moving
club shaft with a highly reflective or white paint.

Finally, one of the major problems associated with making photographs with the DC260
and I assume with other consumer grade cameras as well, is the time delay
involved between the pressing of the shutter release and the actual start of
the exposure or opening of the shutter. Unlike regular cameras, this delay in
digital cameras can amount to as much as a second and it makes it very
difficult to synchronize and event and the operation of the camera's shutter.
Fortunately the recording medium is reusable but still, it is frustrating not
to be able reliably predict the start of the exposure. Often one needs to
anticipate with too much lead time and this leads to unpredictable results.

Moving film technique

Traditional stroboscopic techniques generally have concentrated on simply
opening the shutter at the beginning of the action and closing it at the end
and recording the moving subject during the process. It becomes quickly evident
that this approach has limits in terms of the length of time during which
any given can be recorded because if the time is extended too far, too many
images will superimpose on each other and it becomes impossible to determine
the development or sequence of the action being investigated or visualized.

A method that allows one to examine a moving subject over an extended period
of time is an extension of the techniques described above and it almost
resembles a motion picture camera in its operation but the resulting images are
displayed as flat, two dimensional, photographs instead of time-based, motion
pictures that can usually not be reproduce in text form.

The basis for this extension of the stroboscopic technique depends on
devising a way to place the subject on different locations on the film as the
action develops or progresses. This places those parts of the subject that
essentially remain motionless on different locations on the film and it also
displaces the moving portions of the subject to other locations than those they
would occupy if the subject exposed essentially the same location on the film
with each exposure given by either the flashing light or the rotating slotted
disc.

One way to achieve the displacement of the overall subject across the film
is to rotate the camera or to laterally displace the stage upon which the
subject performs its action across the field of view of the camera.

A very elegant solution to this same problem is to
place the film in motion while the subject is moving and the stroboscopic
light is flashing or the slotted disc is rotating in front of the camera's
lens. A 35mm camera can be easily adapted to the task of moving the film by
first advancing the film to the take-up chamber. This can be done by placing a
lens-cap over the lens and then firing off the camera until the film supply is
exhausted and is all located in the take-up side.

Now, the action stage is set and the lens aperture determined as explained
above. The camera is placed on a tripod and aimed at the scene where the action
will take place. At this time the shutter of the camera, set to "B", is tripped
and locked in the open position with a locking cable release. Now the rewind
release button is pushed up or activated, disengaging the sprocket drive from
the internal gears in the camera. This frees up the film for rewinding. The film
is now simply rewound back into the supply chamber.

Note that some cameras do
not allow rewinding the film while the shutter is locked in the open position.
These cameras are obviously not suitable for this technique. Also, another word of advice
is this: if the camera seems to resist allowing you to rewind the film do not
force the rewind knob. It might break if you apply enough force. Do not force
the mechanism to do something it clearly seems to resist.

It is evident that periodically the slot of the rotating disc flashes an image onto the
film, fixing the position of the subject at that time, and in the time the slot
makes another turn, the subject changes position. In the same time, film has a
chance to move within the camera so that when the slot again flashes an image
of the subject onto the film, the image is recorded slightly offset from the
first one. This process continues until the whole roll of film has passed
through the camera. The result is typically a long series of exposures
depicting the changing positions of the subject over a substantial period of
time. The number of images thus recorded and the separation between them is
now a function of the rotation rate (or flashing rate) of the stroboscope and the
rate at which the film is transported through the camera by the rewind knob.

There is a "rule-of-thumb" relationship that allows a photographer to
determine the length of time it should take to make the rewind knob turn once
given the desired separation of images on the film and the rotation rate (or
flashing rate) of the stroboscope.

It goes something like this: the time to turn the rewind knob once is equal
to 50 divided by the product of the desired separation between images and the
rotation rate of the slotted disk. (If you cut more than one slot in a disk
then the disc rotation rate needs to be multiplied first by the number of slots
cut into it.)

For example, assuming that the disc makes 5 turns each second and that we
desire our images of the sequence to be about 4 mm apart (this would place
about 9 images within a 36 mm length of film, or about 1 standard "frame") one
would need to turn the rewind knob in 50/4x5 or 50/20 or about 2.5 seconds.
For the images to be closer together the time to turn the rewind knob must be
lengthened (the knob turned more slowly) or the disc must be turned more
quickly. In any case, this does not mean that you will be turning the rewind
knob only once! You essentially will be turning the rewind knob until you run
out of film and will be generating an record of the action that extends from
one end of the film all the way to the other, without frame lines or individuals
"frames". A continuous "time" record of discrete instants in time, each showing
your subject in different positions.

Make sure to inform you photofinisher that you are having special film
developed and that it should not be cut into individual 35mm frames. It will be
up to you to locate and identify interesting sections on the film and printing
or saving those.

To print longer sections than those accomodated in a 35 mm enlarger you can
enlarge the images in sections and assemble a long strip or you can print
lengths of film up to about 5 inches long by using a 4x5 enlarger. It is best to use a
glass carrier to hold the film flat and it is highly advisable to mask the
carrier down to a 1 inch wide by 5 inch long opening so as to prevent enlarger
flare from affecting your print's highlights or shadows (depending on whether
you are printing negative or positive materials).

An alternative is to input or scan the film images into a computer using a
slide scanner that can scan long lengths of film and then printing the image
files as a long print. You might also scan insections and assemble a long image
file. Or, a truly esoteric alternative is to find a "strip"
enlarger that accomodates long lengths of film and prints them onto moving
paper.

Finally, the point to remember is that we should not think that we are limited to
taking stroboscopic pictures onto stationary film, but that it is possible to
to explore "time" by the simple expedient of setting the film in motion!

Andrew Davidhazy is a Professor of Photography in the Imaging
and Photographic Technology department of the School of Photographic Arts and
Sciences at the Rochester Institute of Technology. His teaching centers
on instruction related to the use of photography a a tool of measurement
and visualization for researchers, scientists, engineers and technicians.
Among the topics included in his laboratory's activities are infrared and
ultraviolet photography, high speed and time lapse, panoramic and
peripheral photography, low level aerial photography and close range
photogrammetry, thermography, photographic documentation, etc..

You can contact him at RIT at andpph@rit.edu
if you have any questions about the article.

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